In Vivado 2016.3, support was added to the 25G and 50G Ethernet Subsystems for Kintex and Zynq UltraScale+ -2LV devices.
However, support for this speed grade has not been added yet for the 25G or 50G RS-FEC. The 25G and 50G Ethernet Subsystem GUI allows the RS-FEC option to be selected, but will generate an error:
The 25G and 50G RS-FEC is supported on Virtex UltraScale+ -2LV devices and other supported devices.
Support for Kintex and Zynq UltraScale+ -2LV devices is being investigated for future software releases.